Fix PR17493, attempted output of *GAS `reg' section* symbol

Fix PR17493, attempted output of *GAS `reg' section* symbol

The write.c change is to make gas report an error if reg_section
symbols should leak in future.  The tc-i386.c change is the real fix.

Note that the error isn't the most helpful, "redefined symbol cannot
be used on reloc", but I'm not inclined to improve what is really an
internal gas error.  reg_section symbols shouldn't leak..

gas/
	PR 17493
	* write.c (adjust_reloc_syms): Don't allow symbols in reg_section
	to be reduced to reg_section section symbol.
	* gas/config/tc-i386.c (i386_finalize_immediate): Reject all
	reg_section immediates.
gas/testsuite/
	* gas/i386/inval-equ-2.l: Adjust.